The objectives of this proposal are to study the molecular aspects of the complement fixing site on human IgG1. It is proposed to study the ability of some aggregated small peptides of this immunoglobulin to fix complement. From these results it is expected to be able to construct small non-antigenic peptides which will be able to inhibit complement by the classical pathway without the concomitant activation of complement by the alternate pathway. It is hoped that such an approach to inhibition of the complement system will be of direct application to human clinical diseases.